On account of the progressive reduction of the feature sizes to be formed in semiconductor fabrication, a transition to so-called EUV lithography (EUV: extreme ultraviolet) should be expected in the near future, starting at a feature size of approximately 40 nm or less. EUV lithography makes use of light or radiation having wavelengths in the range of 11 to 14 nm in order to transfer the patterns formed beforehand on masks (EUV masks) onto a substrate (e.g., a semiconductor wafer). The EUV radiation is also referred to as soft X-ray radiation.
Besides the requisite conversion of the resist process, this transition from optical projection lithography used at the present time (deep ultraviolet DUV, 284 nm or 193 nm; far ultraviolet FUV, 157 nm) has the consequence that, owing to the nontransparency of lens systems of conventional materials with respect to the EUV radiation, only mirrors or reflection optical systems can be used in the corresponding exposure devices. Therefore, the beam paths of such devices change considerably. Furthermore, reflection masks should preferably be used in this technology since the conventional quartz masks as transmission masks are also nontransparent with respect to the EUV radiation.
The progressive development furthermore consists in providing materials for the components, such as mirrors, masks, etc., which exhibit no or only insignificant degradation with respect to the high-energy EUV radiation. As long as these degradations have a homogeneous effect over the image field in the case of the mirror components, a reduction of the degree of reflection (the reduction being caused for instance by removal on account of the source particles at the mirror surface) can be compensated for in the simple case by a higher intensity of the radiation source, for example, if the scattering effects that possibly accompany this as a result of the degraded mirror surface can be disregarded here.
Degradations may occur in an inhomogeneous manner, however, precisely at the mirror arranged closest to the radiation source, namely the collector mirror. On the one hand, the local radiation intensity is strongest here and, on the other hand, the collector mirror, since it has to couple out the radiation in a specific direction (e.g., as a parabolic mirror) has a form which necessitates different distances between its mirror surface and the radiation source. As a result, different positions at the mirror surface degrade to different extents, under certain circumstances.
The position of the collector in the beam path of the EUV exposure device is essentially conjugate with respect to the position of the mask. Consequently, inhomogeneities in the degree of reflection of the collector mirror also lead to a nonuniform illumination of the image field of the mask. Fluctuations of the line widths of structure elements that are imaged onto the substrate and are processed there in the resist may be the consequence.
Other components, in particular mirrors in the beam path, may be detrimentally affected and degrade over time, such that an inhomogeneous image field distribution may be the consequence. Previous solutions provide for cost-intensively exchanging the corresponding components upon establishing the cause (i.e., the degradation).
Fluctuations of the line widths on the substrate may also be caused, however, by variations of the widths of the corresponding lines or structure elements on the EUV mask which are present from the outset or arise only in the course of time and of the hard production conditions in the EUV exposure device.
In the case of the EUV masks, besides new production, it would also be possible to perform locally limited repairs on the mask. Large-area applications are ruled out here, however. Moreover, in the case of repair, it is necessary to consider the loss of time for returning the mask to the manufacturer and the suddenly occurring reconfiguration with a subsequent product for the exposure device if the degradation is ascertained only on the end product.
Therefore, there is a need to reduce the influence of degradations of the components in the EUV exposure device on the quality of the exposure process and furthermore to reduce the costs for maintaining quality in semiconductor fabrication via EUV lithography.